1. Field of the Invention
The present invention relates to an asymmetric digital subscriber line (ADSL) system, and more particularly, to a method of establishing a communication link in the ADSL system.
2. Description of the Related Art
An asymmetric digital subscriber line (ADSL) system generally comprises a central office (CO) such as a telephone office and customer premises equipment (CPE). The CO and the CPE may include their own corresponding ADSL modems.
In order to establish a communication link in the ADSL system, the following initialization process is performed during a boot operation of the ADSL modem.
Phase locking is initially obtained by a phase locked loop (PLL) in response to an initial pilot tone output from the CO. An equalizer and echo canceller are subjected to learning or training in response to a reception reverb signal, which is a cyclic signal output from the CO.
Next, a signal-to-noise ratio (SNR) of the ADSL system is calculated in response to a medley signal output from the CO. An appropriate downstream data rate is determined based on the calculated SNR. The determined downstream data rate is transmitted from the CPE to the CO. An upstream data rate is similarly determined using the same method as the method for determining the downstream data rate. The upstream data rate is transmitted from the CO to the CPE.
A process for exchanging initialization messages including the data rates is performed in a 4 quadrature amplitude modulation (QAM) system using 4 message transfer tones (hereinafter, referred to as a “message tone”) and 4 associated backup tones. The backup tones have the same initialization messages as the message tones. Each of the initialization messages comprises 2-byte cyclic redundancy checking (CRC) messages.
On the other hand, the message tones and the backup tones may be affected by noise such as a high bit-rate DSL (HDSL) crosstalk and European Telecommunication Standards Institute (ETSI) FA/FB/FC/FD crosstalk.
The ETSI 388 test specification is a European standard specification. In the test specification, 8 types of test loops are specified. In addition, it is specified that the desired test rate is generated by applying the ETSI FA/FB/FC/FD crosstalk to the test loops while increasing the lengths of the test loops.
If the ETSI FA or FB crosstalk is applied to the test loops and the lengths of the test loops increase, the process for establishing the communication link in the ADSL may not be successful. For example, in a case where the ADSL system modem attempts to perform the process for establishing the communication link on a test loop of 26 American wire gauge (AWG) having a length of 2950 m by using an European standard ADSL annex B mode, if the ETSI FB crosstalk is applied to the test loop, the process for establishing the communication link in the ADSL system may not be successful. In addition, in a case where the ADSL modem attempts to perform the process for establishing the communication link on a test loop of 26 AWG having a length of more than 2950 m by using the ADSL annex B mode, if the ETSI FA crosstalk is applied to the test loop, the process for establishing the communication link in the ADSL system may not be successful.
FIG. 1 depicts the HDSL crosstalk that occurs in an American standard ADSL annex A mode. Referring to FIG. 1, a frequency response of the HDSL crosstalk is depicted.
As shown in FIG. 1, noise power of the HDSL crosstalk is relatively large in the low frequency band. Since noise such as the HDSL crosstalk affects the message tones and the backup tones in the low frequency band, errors may occur in the initialization message transmitted or received through the message tones and the backup tones. In turn, the erroneous initialization messages may generate a link failure in the ADSL system.
FIG. 2 depicts the ETSI FB crosstalk that occurs in a European standard ADSL annex B mode. Referring to FIG. 2, a frequency response of the ETSI FB crosstalk is depicted. Locations of the message tones and the associated backup tones in the ADSL annex B mode are indicated by A and B.
Similar to the HDSL crosstalk shown in FIG. 1, noise power of the ETSI FB crosstalk is also relatively large in a low frequency band. Therefore, as described above, the erroneous initialization messages may generate link failures in the ADSL system. In other words, in the ADSL system of the ADSL annex B mode, as shown in FIG. 2, since the initialization messages (for example, C_RATESRA, C_MSGRA, CRATES2, C_MSG2, and C_B&G) and the CRC messages associated with the initialization messages are transmitted in the 4 QAM system using the 75-th to 78-th message tones and the 91-th to 94-th backup tones in the low frequency band, a large number of errors in the initialization message occur due to the ETSI FB crosstalk, so that a link failure may be generated. In addition, since an ADSL system of the T1.413 annex A mode utilizes the 37-th to 40-th backup tone in a lower frequency band, the backup tones may be further affected by a crosstalk such as the ETSI FB crosstalk. Therefore, a large number of errors in the initialization messages occur due to the crosstalk, so that it may be impossible to correct the errors.
FIG. 3 is a table illustrating an example of C_RATESRA and C_MSGRA messages in the message tones in which errors occur due to the ETSI FB crosstalk. FIG. 4 is a table illustrating an example of transmitted/received C_RATESRA and C_MSGRA messages in the backup tones in which errors occur due to the ETSI FB crosstalk, wherein the transmitted/received C_RATESRA and C_MSGRA messages correspond to the transmitted/received messages in the message tones shown in FIG. 3.
Referring to FIGS. 3 and 4, there are listed C_RATESRA and C_MSGRA messages of the initialization messages generated in the case where ETSI FB crosstalk is applied to the ETSI test loop 1 having a length of 2950 m.
In the tables shown in FIG. 3 and 4, “OX” at each cell denotes hexadecimal. The C_RATESRA and C_MSGRA messages have 2-byte CRC messages at their own end portions. The C_RATESRA and C_MSGRA messages are transmitted in a fast mode of operation of the ADSL system. A frame contained in each of the messages has 1-byte information.
As shown in FIGS. 3 and 4, the C_RATESRA and C_MSGRA messages transmitted/received by the message tones and the backup tones have a large number of errors due to the crosstalk. The errors take the form of different data values that are received during respective transmissions of the same data using the message tones (FIG. 3) and the backup tones (FIG. 4). Even though the initialization messages of the message tones and the backup tones are compared and checked in this manner, it is impossible to determine which of the tones comprise the initialization messages having a small number of errors. Therefore, the process for establishing the communication link in the ADSL will not be successful due to the large amount of errors. These errors, which lead to link failures, are indicated in the tables of FIGS. 3 and 4 as shaded cells.